Carbon substances having nanometer-scale microstructures, such as single-walled or multi-walled carbon nanotubes, carbon nanohorns, fullerenes, and nanocapsules, have recently attracted much attention. Such carbon substances are expected to find applications as nanostructured graphite substances for new electronic materials, catalysts, and optical materials. In particular, carbon nanohorns have attracted attention as substances that are the closest to practical use in electrode materials or gas-adsorbing materials of fuel cells.
An arc discharge method, a chemical vapor deposition (CVD) method, and a laser ablation method have been used for producing such carbon nanomaterials. In particular, a variety of methods for producing carbon nanoparticles by arc discharge has been disclosed because nanotubes produced by the arc discharge method have few defects of atomic arrangement (for example, Patent Documents 1 to 5). With those methods, carbon nanomaterials are formed from carbon by vaporizing carbon in vacuum, air, or liquid nitrogen. A method for generating carbon nanohorns by generating carbon vapor by arc discharge in water and rapidly cooling the carbon vapor has also been suggested (for example, Non-patent Document 1).